1. Field of the Invention
The present invention relates to an escapement, and to a timepiece movement and a timepiece which include the escapement.
2. Description of the Related Art
In general, a mechanical timepiece includes an escapement for controlling rotation of a barrel wheel, a center wheel & pinion, a third wheel & pinion and a second wheel & pinion which configure a front train wheel. The escapement mainly includes an escape wheel, a double roller disposed in a balance which pivotally moves around a balance staff, and a pallet fork which is pivotally movable around a pallet staff.
The double roller includes an impulse pin which comes into contact with the pallet fork, and pivotally moves together with the balance by using energy accumulated in a hairspring. The pallet fork includes an entry pallet and an exit pallet which can disengage from a tooth portion of the escape wheel, and pivotally moves around the pallet staff by the energy of the hairspring being transmitted to the pallet fork via the impulse pin.
If the pallet fork pivotally moves around the pallet staff, the entry pallet and the exit pallet alternately disengage from the tooth portion of the escape wheel. When the entry pallet and the exit pallet of the pallet fork engage with the tooth portion of the escape wheel, rotation of the escape wheel is temporarily stopped. In addition, when the entry pallet and the exit pallet disengage from the tooth portion of the escape wheel, the escape wheel is rotated. These operations are continuously repeated so that the mechanical timepiece tells time.
Incidentally, in general, the energy of the hairspring is provided from a mainspring accommodated in the barrel wheel via the front train wheel and the escapement.
For example, European Patent Application Publication No. 0018796 (Patent Document 1) discloses the escapement including the escape wheel having an escape pinion, the pallet fork having the entry pallet, the exit pallet and a third pallet (pallet 26), and the double roller having a first impulse pin and a second impulse pin (pallet 25). According to a technology disclosed in Patent Document 1, when the balance and the double roller pivotally move clockwise, the tooth portion of the escape wheel comes into contact with the second impulse pin so as to provide the hairspring with the energy. In addition, when the balance and the double roller pivotally move counterclockwise, the escape pinion of the escape wheel comes into contact with the third pallet so as to provide the hairspring with the energy via the pallet fork.
However, in the related art, it is necessary to dispose the third pallet coming into contact with the escape pinion in the pallet fork. Therefore, there is a tendency that the pallet fork is increased in size. In addition, in order to enable the escape pinion and the third pallet to come into contact with each other, there is a tendency that the escape pinion is increased in size. This increases viscosity frictional resistance when the pallet fork and the escape wheel pivotally move. In addition, since the increased size of the pallet fork and the escape wheel increases a weight thereof, solid frictional resistance in the pallet staff or a bearing of an axle portion of the escape wheel increases. This increase in the viscosity frictional resistance and the solid frictional resistance causes energy loss in the escapement.
Furthermore, the increased weight of the pallet fork and the escape wheel increases the moment of inertia. Consequently, a movement becomes slow when the pallet fork and the escape wheel pivotally move. This narrows an impact range when the pallet fork and the escape wheel collide with the impulse pin, and thus it is not possible to efficiently transmit the energy.
As described above, in the related art, there is a problem in that energy transmission efficiency in the escapement needs to be improved by virtue of the increased size and the increased weight of the pallet fork and the escape wheel.